Security elements are typically applied to security documents such as bank notes, ID cards, certificates, checks, credit cards and the like in order to secure the security document against falsification and/or adulteration. Typical security elements include watermarks, perforations, security strips, holograms and so on, which are very difficult to reproduce. In general, security documents are provided with a number of security elements, some of which can be detected with the naked eye, and some of which can only be detected using external testing equipment, for example by viewing through incident light, transmitted light or ultraviolet light.
Documents U.S. Pat. No. 5,582,103 and EP 1 325 816 A2 disclose security elements for security documents, the elements revealing two different pieces of information by way of a specific combination of embossed line grids and printed line grids under two different angles of observation. When viewed straight on, the known prior art security element appears as a uniform gray surface, for example, whereas when viewed at an angle from a first direction a gray line appears on a bright background. Upon viewing at an angle from a second direction, the gray line appears on a dark background.
A disadvantage of the disclosed security elements is the relatively low amount of information contained therein.